Three important attributes of an image sensor are resolution, sensitivity and in many cases frame rate. Higher sensor resolutions (more pixels) generally produce images with finer detail whereas higher sensitivity enables the sensor to better resolve faint scenes with acceptable signal to noise ratios. An image sensor with a higher frame rate enables the ability to capture multiple images within a close time period and is particularly useful in scenes with object movement.
For a given optical format, resolution is generally improved by decreasing pixel size. Conversely, sensitivity is generally improved by increasing pixel size. Thus, resolution and sensitivity are inversely proportional for a given optical size. Furthermore, higher resolution negatively impacts frame rate given fixed pixel readout rates.
What is desired is an effective method for trading off resolution for improved sensitivity and frame rate on a color image sensor with the typical color arrangement of a Bayer color filter array.
A common method for improving the sensitivity of an image sensor is to add the signal from adjacent pixels typically of the same color type. If performed off-chip (digitally), the resulting signal-to-noise (SNR) is only improved as:
                              SNR          off                =                                            n                        ⁢            S                                              S              +                              σ                2                                                                        [        1        ]            where S is the signal of a single pixel, σ represents a fixed amplifier noise and n is the number of pixels combined together. The term sqrt(S) is the photon shot noise. This relationship assumes that all pixels have the same signal level and other noise sources, such as dark current shot noise, clocking noise, off-chip electronics and analog-to-digital (ADC) noise, are negligible compared to the sensor amplifier noise. This is commonly the case for a well-constructed camera system. The improvement in SNR reduces the resolution by a factor of 1/n but frame rate remains unchanged.
In the present invention, signals are added from adjacent pixels of the same color type within the sensor. In this case, the resulting signal-to-noise (SNR) is improved by:
                              SNR          on                =                  nS                                    nS              +                              σ                2                                                                        [        2        ]            The effect of the amplifier noise is reduced—particularly at low signal levels where the photon shot noise is low compared to the amplifier noise.
In US Patent Application Publication US 2006/0109352 A1 by Draijer, a structure is disclosed for combining pixels on-chip as discussed. In this case, it requires an additional temporary storage register with associated control lines to achieve the desired result. This extra structure adds to chip size that, in turn, lowers yield resulting in a higher cost device. In addition, the risk of charge transport errors, due to inherent manufacturing defects, is increased by requiring pixels to transfer both up and down as well as left and right in the process of combining pixels. This latter point is described in detail by Janesick in “Scientific Charge-Coupled Devices”, SPIE Press, 2001, chapter 5.4 and illustrated on pg. 449-450. Lastly, once the process for combining pixels is completed, time is required to transfer pixels out of the temporary storage register into the readout register leading to a reduction in frame rate.
Consequently, a need exists to overcome the above-described drawbacks.